1. Field of the Invention
The present invention concerns a magnetic resonance apparatus and method to generate a gradient field in a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance apparatuses, in particular to examine patients by magnetic resonance tomography, are known from DE 103 14 215 B4, for example.
Modern magnetic resonance systems operate with coils that emit radio-frequency pulses for nuclear magnetic resonance excitation and/or to receive induced magnetic resonance signals. A magnetic resonance system typically has a larger coil that is normally permanently installed in the apparatus (what is known as a whole-body coil, also called a body coil or BC) and multiple small local coils (also called surface coils or LCs).
To obtain information (data) from which images can be generated, selected regions of the subject or, respectively, patient to be examined can be read out with gradient coils for three axes (for example X, Y, approximately radial to the patient, Z in the longitudinal direction of the patient).
The spatial coding in magnetic resonance tomography is typically achieved with the use of a gradient coil system with three independently controllable, magnetically orthogonal gradient field coil systems.
The orientation of the coding plane (“gradient field”) can be freely selected by superimposing the three freely scalable fields (in three directions X, Y, Z). In some important MR applications—for example EPI (echoplanar imaging)—this degree of freedom is utilized only to a limited degree. Here the horizontal axis (X-axis) is always used for the readout gradient, which is also designated as a clinically established standard or “gold standard”. In EPI the primary load of the gradient sequence (>90% of the sum power of all axes) lies on this horizontal axis (X-axis). In modern gradient systems, the coordinate axes of the three field coils are oriented parallel to the patient coordinate system. Given an axially parallel, asymmetrical gradient load, this leads to a power limitation due to the hardware needed for a single axis. The EPI readout amplitudes are typically limited by the available cooling power for the gradient field coils of the X-axis of the gradient system.
This problem has previously been addressed by dimensioning the system limit according to the limit, for a single axis, and corresponding limitation of the sequence parameters.